Green preparation of graphene oxide nanosheets as adsorbent

As a basic building block of graphene-based materials, graphene oxide (GO) plays an important role in scientific research and industrial applications. At present, numerous methods have been employed to synthesize GO, there are still some issues that need to be solved, thus it is of importance to develop a green, safe and low-cost GO preparation method. Herein, a green, safe and fast method was designed to prepare GO, namely, graphite powder was firstly oxidized in a dilute sulfuric acid solution (H2SO4, 6 mol/L) with hydrogen peroxide (H2O2, 30 wt%) as oxidant, and then exfoliated to GO by ultrasonic treatment in water. In this process, H2O2 was the only oxidant, and no other oxidants were used, thus the explosive nature of GO preparation reaction in the conventional methods could be completely eliminated. This method has other advantages such as green, fast, low-cost and no Mn-based residues. The experimental results confirm that obtained GO with oxygen-containing groups has better adsorption property compared to the graphite powder. As adsorbent, GO can remove methylene blue (50 mg/L) and Cd2+ (56.2 mg/L) from water with removal capacity of 23.8 mg/g and 24.7 mg/g, respectively. It provides a green, fast and low-cost method to prepare GO for some applications such as adsorbent.

www.nature.com/scientificreports/ graphite with a direct current of 5 V, and finally hired ultrasonic to exfoliate the intercalated oxidized graphite. In this process, H 2 SO 4 was primarily used as a control agent to tune the oxygen evolution reaction of water for the oxidation of graphitic lattice, thus H 2 SO 4 was recycled, and no other oxidant was needed. Unfortunately, the accompanied water/solvent electrolysis process aggravates the expansion and delamination of graphitic materials, which lead to ineffective current supply or broken circuit 29,30 . Moreover, the structure and property of obtained products are very different from that of GO prepared by the improved Hummers' methods [5][6][7][8][9][10] , maybe due to the low oxidation and exfoliation degree [32][33][34][35] . At the same time, great efforts have been paid to uncover the formation mechanism of GO 36 . For example, Lee et al. 37 exfoliated and dispersed 2D materials in pure water due to the presence of surface charges as a result of edge functionalization or intrinsic polarity, which can induce the oxygen attacking. Li et al. 38 exfoliated large graphite crystals to small graphene flakes due to the sonication leading to the rupture and kink band striations on the flake surfaces, subsequent oxidative attack and intercalation of solvent. More recently, Zhu et al. exquisitely prepared GO sheets by the microfluidic oxidation of graphite with KMnO 4 in concentrated H 2 SO 4 in a sealed screw-top bottle due to the enhanced mass transfer and extremely quick energy exchange 39 .
On the basis of the development of GO preparation methods and the exploration of GO formation mechanism above mentioned, we are trying to find a green, safe and low-cost way to prepare GO. In this paper, a green, safe and low-cost method was designed to synthesize GO by oxidizing graphite powder in a dilute H 2 SO 4 solution with H 2 O 2 as oxidant, and subsequent ultrasonic treatment in water. The experimental results prove that GO can be prepared by this method. Interestingly, the obtained GO with oxygen-containing groups can be used as adsorbent to remove methylene blue and Cd 2+ from water.

Experimental section
Materials. Graphite powder (99.8 wt% purity, 200 mesh), H 2 SO 4 (18 mol/L) and H 2 O 2 (30 wt% in water) were purchased from National Pharmaceutical Reagent Company. All chemicals were used without further purification. Deionized water (a resistance of 18 MΩ) made from a Milli-Q solvent system was used through all the experiments.

Preparation of GO.
As a typical process, the graphite powder was firstly oxidized in a mixture of H 2 SO 4 and H 2 O 2 , and then exfoliated to GO in water.
Firstly, 2.0 g of graphite powder was added into a beaker with 50 mL of dilute H 2 SO 4 solution (6 mol/L), and 20 mL of H 2 O 2 (30 wt%) was slowly added into the mixed dispersion under electromagnetic stirring condition. Then, the mixed dispersion was heated to 40 ℃ through a temperature control water bath for 3 h. As control experiments, another beaker was added with 2.0 g of graphite powder and 50 mL of H 2 SO 4 solution without adding H 2 O 2 , following the same experimental operation as that of the first beaker.
Secondly, after cooling naturally to room temperature, the resulting dispersion was filtered and cleaned with deionized water several times to pH of 7. The dilute H 2 SO 4 solution was collected and stored for reuse.
Thirdly, a portion of sample was treated with ultrasound in deionized water for 30 min, after filtering, the obtained GO was dried in vacuum at 60 °C for characterizations.
In this process, only H 2 O 2 was oxidizer, and no other oxidant was used. Dilute H 2 SO 4 solution was primarily used as an acidic medium 28,31 and control agent to tune the oxygen radical reaction, thus H 2 SO 4 was recycled. The preparation process takes less than 4 h, including 3 h of water bath at 40 °C for oxidation reaction and 30 min of ultrasonic stripping at room temperature.

Results and discussion
Structure analysis. The microstructures of graphite powder, oxidized graphite (marked as intermediate) and GO are shown in Fig. 1. The graphite powder has irregular and tightly stacked structure (Fig. 1b), while the structure of intermediate is loose (Fig. 1c), very different from that of graphite powder, and it can be easily scattered in the deionized water (the inset of Fig. 1c). After ultrasound treatment in deionized water, the intermediate can be exfoliated to GO (Fig. 1d), and the color of GO solution is light yellow (the inset of Fig. 1d). Figure 2a further reveals that graphite powder has irregular and stacked structure with sizes in a range of dozens of microns. In contrast, the intermediate has a loose structure with smaller sizes, as shown in Fig. 2b. Moreover, the cross-section SEM images of graphite powder and intermediate show their difference more clearly. Compared to graphite powder (Fig. 2c), the intermediate (Fig. 2d) has wider layer spacing. After ultrasonic process, the graphite powder can be separated into large sheets with a size of several microns (Fig. 2e), while the intermediate can be stripped into GO nanosheets (Fig. 2f).
TEM images reveal that obtained GO with folded structure is single-layer or multilayer, as shown in Fig. 3. Thus, it can be confirmed that GO nanosheets can be prepared from graphite powder by this method.
After the intermediate was exfoliated to an aqueous solution of GO (0.5 mg/mL) by ultrasonic treatment, and then GO solution was dripped onto freshly cleaned Si substrate after vacuum drying for AFM characterization. Figure 4 shows that GO with a wrinkled structure has a few-layer thickness 40  www.nature.com/scientificreports/ An aqueous GO solution (1.0 mg/mL) has a clear Tyndall effect (Fig. 5a). UV-Vis spectrum of this GO solution has a main peak at 231 nm and a broad shoulder at around 300 nm (Fig. 5b), suggesting that some oxygencontaining functional groups are bonded to the basal planes and edges of GO 24 .
XRD, FTIR and Raman spectra of graphite powder and obtained GO nanosheets were analyzed to reveal their structural difference. As shown in Fig. 6a, the XRD peak of graphite powder locates at 2θ = 26.4°, while the peak of GO nanosheets locates at 2θ = 10.0°, and their peaks are consistent with previous reports 41, 42 , indicating a larger spacing between GO layers compared with graphite powder.
Raman analyses were also used to prove their structural change (Fig. 6c). It can be seen that both of them have D peak (∼ 1320 cm −1 ), G peak (∼ 1580 cm −1 ) and 2D band (∼ 2650 cm −1 ). Moreover, the intensity ratio of the D band (∼ 1320 cm −1 ) to the G band (∼ 1580 cm −1 ) (I D /I G ) slightly increases from 0.38 for graphite powder to 0.45 for the obtained GO nanosheets, indicating that the crystal defects and disorder are increased in the obtained GO nanosheets 41,42 . Moreover, their 2D Raman peak fittings have clear difference, as shown in Fig. 6d.
The XPS spectra of graphite powder and GO nanosheets were performed to analyze their composition. As shown in Fig. 7, both XPS survey spectra have two clear peaks of carbon (C1s) and oxygen (O1s), while the intensity of the O1s peak in GO increases significantly, compared with graphite powder (Fig. 7a). The highresolution C1s spectra reveal that both of them have C=C/C-C (284.7 eV), C-O (285.8 eV) and C=O (286.7 eV) groups 41,42 , respectively. Their high-resolution O1s XPS spectra have two major peaks located at 531.8 eV (C-O) and 533.0 eV (C=O), respectively. Moreover, the peak intensity of C=O for graphite powder (Fig. 7b,c) is lower than that of GO nanosheets (Fig. 7e,f), maybe due to the oxidation intercalation of H 2 O 2 .

Mechanism analysis.
To analyze the preparation process of GO by this method, some important factors should be taken into account as follows.
(1) In the system of H 2 SO 4 /H 2 O 2 , only H 2 O 2 acts as a strong green oxidizer, and a dilute H 2 SO 4 solution (6 mol/L) is an acidic medium. In this process, some oxygen radicals (such as ·OH and ·OH 2 ) can be produced from H 2 O 2 under acidic conditions, and then the creases and defects on the graphite surface are selectively attacked by oxygen radicals 37,38 . As a result, the selective attacking endowed GO with some oxygen-containing groups, which are confirmed by the FTIR and XPS spectra of graphite powder and GO nanosheets. Dilute H 2 SO 4 solution is primarily used as an acidic medium 28,31 and control agent to tune the oxygen radical reaction, thus H 2 SO 4 is recycled. (2) The oxidant content and the reaction temperature are very critical influencing factors. In this process, if the oxidant content is low, and the desired oxidation effect will not be achieved. On the other hand, if the   www.nature.com/scientificreports/ groups. The delamination was completed by ultrasonic treatment in deionized water, and no other intercalation agent was required in this process.
According to the above analysis, only H 2 O 2 was used as oxidant without harmful byproducts, the dilute H 2 SO 4 solution was used as acidic medium and recycled. The dispersion containing intermediate was not viscous, and the intermediate was easily filtered and cleaned for delamination, thus the preparation process was green, safe and low-cost.
The microstructure of the intermediate was analyzed to confirm the feasibility of this method. As shown in Fig. 8a, the intermediate shows a loose structure, compared with graphite powder (Fig. 1b). The magnified parts of its selected area clearly display some traces after selective attacking (Fig. 8b-d). Moreover, the spacing between layers was markedly widened in the intermediate (Fig. 8e), and lots of traces were left at the edges after selective attacking (Fig. 8f), thus GO nanosheets were successfully prepared from graphite powder to intermediate after the oxidation intercalation of H 2 O 2 and ultrasonic treatment in deionized water.
Adsorptive property. The obtained GO nanosheets with abundant functional groups can be used as adsorbent to remove methylene blue (MB) and Cd 2+ from water. In general, MB was used as the model compound to verify the adsorption property of adsorbent. The general experimental process was described as follows: 110 mg of adsorbent was added to 50 mL of MB solution (50 mg/L), followed by stirring at 200 rpm at room temperature. At time intervals of 10 min, the adsorbent was separated by filtration and the concentration of residual MB in the filtrate was calculated by Beer's law based on the absorption peak at 662 nm with a UV-Vis spectrophotometer (Shimadzu UV-2450) 43,44 .
On the other hand, an aqueous solution of Cd 2+ was prepared by dissolving the corresponding lead nitrate in deionized water to arrive at a concentration of 5.0 × 10 -4 mol/L (56.2 mg/L). Adsorption tests were carried out by using 130 mg adsorbent in 50 mL aqueous solution of Cd 2+ . Batch experiments of adsorption were carried out in conical flasks with stirring under ambient condition. At given time intervals of 10 min, 1 mL of the filtrate was obtained by filtering from the mixed solution. The concentration of Cd 2+ in the filtrate was determined by single scan oscillopolarography (JP-303E) 45,46 .
In contrast, the graphite powder and GO nanosheets adsorbed MB (50 mg/L) at room temperature, their adsorption equilibriums reached at around 50 min (Fig. 9a), their removal efficiency are 75.0% and 95.0%, respectively. Moreover, their adsorption capacity were evaluated as 18.8 mg/g and 23.8 mg/g, respectively.    www.nature.com/scientificreports/ On the other hand, the graphite powder and GO nanosheets adsorbed Cd 2+ (56.2 mg/L) from water at room temperature, their adsorption equilibriums reached at around 50 min (Fig. 9b), their removal efficiency were 46.0% and 88.0%, respectively. As well as their adsorption capacity were evaluated as 12.9 mg/g and 24.7 mg/g, respectively.
From the above comparison of adsorption properties, the graphite powder and GO nanosheets can adsorb MB and Cd 2+ from water, especially the adsorption capacity of GO nanosheets is higher than that of graphite powder and some results reported [47][48][49][50][51] . The adsorption results were also compared with the literature [47][48][49][50][51][52][53][54][55] . The adsorption characteristics of MB and Cd 2+ onto GO or GO composites were shown in Table 1. In our experiment, GO was directly used as adsorbent, and its adsorption property should be improved. In particular, GO can be hybridized to fabricate composite materials with better adsorption property. For example, the GO/chitosan sponge with chitosan sponge content of 9% has adsorption capacity of 275.5 mg/g for MB 56 .
Due to abundant functional groups and structure defects on its base planes and edges, GO has unique structure and property (such as novel physicochemical property, large specific surface area, and highly active surface), which plays an important role in the removal of organic and inorganic pollutants from water 57 .
In the case of MB adsorption, negatively charged GO interacts with positive dye of MB, thus the electrostatic attraction and hydrophobic π-π interactions are responsible for this adsorption process 58,59 . Moreover, the adsorption kinetics and thermodynamics of MB onto GO was demonstrated as a mixed physisorption-chemisorption process 60 .
To remove heavy metal ions from water, GO has various interactions such as coordination, chelation, electrostatic interaction, π-π interaction, acid base interaction with various metal/metal ions, due to its unique chemical structure containing various hydrophilic containing-oxygen groups and tiny sp 2 carbon domains surrounded by sp 3 domains [61][62][63] . Bsaed on the literature reported [64][65][66] , the adsorption process of Cd 2+ onto GO was consistent with the pseudo second-order equation and the Langmuir isotherm model. However, the actual adsorption process has various influencing factors 67 , such as the electronegativity and standard reduction potential of heavy metal ions.
On the other hand, the experimental conditions play an important role in the specific adsorption process. In the cases of MB and Cd 2+ , some importan influence factors (such as adsorbent dose, solution pH, contact time and temperature) should be taken into account as follows. As shown in Fig. 9, their adsorption equilibriums reach at around 50 min, and the adsorption contact time is considered to be extended to one hour. In order to save operating costs, the adsorption temperature is kept at room temperature.
At the same time, the dosage of adsorbent is gradually increased from 50 to 70, 90, 110, 130, 150 and 170 mg in 50 mL of MB solution (50 mg/L) or Cd 2+ solution (56.2 mg/L), and the pH of solution is gradually adjusted from 1 to 12. Figure 10a shows that adsorption removal of MB reaches equilibrium as the dosage of adsorbent is 110 mg in 50 mL solution, while Fig. 10b reveals that adsorption removal of MB reaches peak at pH of 10. On the other hand, Fig. 10c shows that adsorption removal of Cd 2+ reaches equilibrium as the dosage of adsorbent is 130 mg in 50 mL solution, while Fig. 10d reveals that adsorption removal of Cd 2+ reaches peak at pH of 7.

Conclusion
In conclusion, we develop a green, safe, fast and low-cost method to prepare GO nanosheets by oxidizing graphite powder in a dilute H 2 SO 4 solution with H 2 O 2 as oxidant and subsequent ultrasonic stripping in deionized water. The formation mechanism can be attributed to strong oxygen radicals attacking to creases and defects on the surface of large graphite sheets, which cause intermediate with oxygen-containing functional groups to be easily exfoliated into nanosheets by ultrasonic treatment. This work provides a green, safe and low-cost method to prepare GO nanosheets for functional applications such as adsorbent.